Developmental neuron death occurs within the superior cervical ganglion during the first two postnatal weeks, and reduces the overall population of principal neurons by 40%. The extent of neuron death is different in males and females, and there is evidence that the gender difference in survival is restricted to a subpopulation of SCG neurons that project out the internal carotid nerve. Neuron death has been attributed to failure to compete successfully for limited amounts of trophic factor at the target. The principal target trophic factor for sympathetic postganglionic neurons in NGF, but other factors have been demonstrated in target tissues that promote sympathetic neuron survival. There is evidence that different subpopulations of sympathetic neurons require different trophic factors for survival, and that the trophic support required may depend upon the target tissue innervated. In addition to target trophic support, the afferent input may play a role in regulating neuron survival. The proposed experiments will study the time course and amount of developmental neuron death within subpopulations of sympathetic neurons identified by 1) the age of their final mitotic division, and 2) the target tissue they innervate. The roles of afferent input and target tissue in the regulation of developmental neuron death will be studied in these subpopulations of neurons. Additional studies will determine whether the postganglionic neurons that have their somas near the small granule containing cells of the ganglion innervate particular target tissues. Ganglia of males and females will be compared to determine whether gender differences exist in the development of any of these subpopulations.